U.S. Pat. No. 5,477,250 issued on Dec. 19, 1995 discloses a direct printing apparatus. The direct printing apparatus includes a rotatable cylinder or toner carrier retaining charged toner particles on its outer periphery, and a backing electrode spaced apart from the toner carrier. The backing electrode is electrically connected to a power source, thereby forming an electric field for attracting the charged toner particles on the toner carrier toward the backing electrode. Interposed between the toner carrier and the backing electrode is an insulating plate having a plurality of apertures through which the toner particles can pass. The insulating plate bears signal electrodes on one surface facing the backing electrode and base electrodes on the other surface facing the toner carrier, and each pair of signal and base electrodes surround the aperture.
In operation, if negatively charged toner particles are used, a positive voltage is charged to the backing electrode. In this instance, when a negative voltage is applied to the base electrode while a positive voltage is applied to the signal electrode, an electric field is formed from the signal electrode to the base electrode, which affords propelling of the negatively charged toner particles through the aperture onto a sheet substrate such as plain paper which is moving past between the insulating plate and the backing electrode. Then, with keeping the voltage applied to the base electrode unchanged, the voltage applied to the signal electrode is changed so that an electric field is formed from the base electrode to the signal electrode, thereby inhibiting an additional propelling of the toner particles.
As described, according to the prior art direct printing apparatus, the propelling of the toner particles is controlled by changing the voltage applied to the signal electrode and thereby reversing the direction of the electric field, in order to form an image of toner particles on the sheet substrate transported between the insulating plate and the backing electrode.
The direct printing apparatus, however, has a drawback that the toner particles tend to diverge in their propelling. Therefore, each resultant dot formed by the toner particles on the sheet substrate is relatively large in size than expected, reducing the density and clearness thereof.